legacycentral bbs

I turned up my MBC a bit today and started hitting fuel cut. But, the bahavior was really weird. The weird thing is, I was getting fuel cut at only about 10psi on the boost gauge (if that). By contrast, my laptop running VRG3's scan tool showed that the ECU was getting erratic boost readings jumping all over from 10psi to 13psi. Is that normal?

I expected to be able to run a stable 11-12psi without tripping fuel cut.

The other weird thing is that the fuel cut only happened about once every 5 seconds. It would cut fuel for a second, but after that the ECU would let the car pull hard again at full boost for a good 5 seconds before hitting fuel cut again. There were never any CEL's, and I didn't have to reset the battery to get it to work right again.

I dropped the boost back down to 7-8 and it doesn't seem to do it anymore.

Basically I want to know if this is normal.

Setup:
* WRX TMIC
* Hallman MBC (plumbed properly between compressor outlet and wastegate actuator)
* Stock turbo, stock fuel system, stock engine management, stock BOV with custom adaptors to fit the intercooler
* Boost control solenoid connected to harnes but disconnected from all tubing.
* Mechanical boost gauge plumbed out of intake manifold pressure(specifically - I tee'd off the vacuum line that runs from the intake manifold to the stock pressure sensor)

I dont know about down your way, but up here the drop from around 60F to 35F does wonderful things--boost comes up faster and even with the Profec B the gauge goes from 12 to 13- 14 that's without dialing up
the max setting.

All else being equal, at cooler ambient temp and WOT the engine demands more fuel, perhaps enough to trigger your cut.

I've got the SDS ecu which eliminated the cut as well as the knock sensor
retard thing. So far, so good.

You do need to sustain an overboost condition for about three seconds to trigger the overboost fuel cut.

The erratic readings bouncing all over the place don't sound right... Have you checked the relevant hoses (from the manifold to the pressure exchange solenoid, and from the solenoid to the sensor) and the sensor's electrical connector?

I will check those things.

I blew the IC hose off the throttle body today during my rallycross because it was assembled wrong and barely clamped at all. Maybe it had been leaking erratically and that was part of the problem. I will turn up the boost again just to check if the fuel cut magically fixed itself when I fixed the coupler.

Normally if you hit fuel cut due to boost, don't you have to reset the battery to get the car to drive right and/or pull hard?

rallysam wrote:Normally if you hit fuel cut due to boost, don't you have to reset the battery to get the car to drive right and/or pull hard?

That hasn't been my experience... but when they're modified to that extent our cars do seem to exhibit varying behaviors anyway so it might be hard to tell.

More investigation

1) The autometer gauge is reading way less than the ECU. Autometer reads 6psi when ECU reads 11-12psi. That's terrible!

2) The ECU pressure reading isn't jumping around THAT much, maybe it just jumps by 1psi. I'm guessing that's normal?

I guess I am leaning towards thinking there is something wrong with the autometer. Either the gauge just sucks that bad or there's a leak in the lines leading to it.

Well, now I really know.

I looked at the autometer before I started the car and it read negative 6psi!!!!

After driving a while then turning the car off, it was a bit better. Now it reads -2 psi. What the???

Hmm. From what you've said, I feel that you probably shouldn't put too much faith in the gauge's reading.

But seriously... I can't really think of what could make a Bourdon tube pressure gauge do that. I guess maybe some part of the linkage is binding against another part? Or a detent failed? I dunno. You could try taking it apart to repair it, but in my opinion it's not worth it since you'd likely ruin what is left of the calibration anyway.

Did you knock the gauge really hard on something??

When I got mine it read off, so I returned it and got another one and it reads great. Sounds like you might have a faulty gauge.

-Ryan

do you have an auto or man tranny? I tried to use an mbc with my auto but had alot of trouble with it so I went to ebc. not sure if it was the 10 dollar home depot style mbc at fault or the auto tranny but I recall overboosting when shifting gears or something to that affect.